1. Field of the Invention
The present invention relates to an optical circuit element and optical circuit device for monitoring light that is propagated in a main waveguide, to a method of fabricating the element and device, and to a method of fabricating a micropyramid mirror for diverting the optical path of light that is transmitted from a waveguide. The micropyramid mirror of the present invention is widely applied as an element for coupling light propagated in a waveguide or optical fiber with a photo-detecting or light-emitting element of a component such as a photodiode or laser diode in mixed optical/electrical circuits that incorporate both optical and electrical elements.
2. Background Art
With the popularization of wavelength-division multiplexing communication in recent years, waveguides in optical circuit devices are also being constructed to enable multichannel communication. It has therefore become extremely important to monitor optical signals, to monitor wavelength, and to monitor power for the purpose of error detection for determining whether the optical signal of each channel is being propagated properly in each waveguide and to check the level of optical power that is being propagated. Methods are currently adopted in which a waveguide directional coupler (for example Japanese Patent Laid-Open No. H10-206911) or a Y-branching waveguide (for example Japanese Patent Laid-Open No. H9-113743) is provided to branch a beam of light, the optical signal that is delivered from the end of the device being coupled to an optical fiber and received by photodiode and then monitored.
In the case of a waveguide device in which the propagated light is transmitted in multiple channels, methods have been adopted in which optical fibers are coupled to a waveguide device, a fiber coupler (for example Japanese Patent Laid-Open Publication No. H6-281837) is provided in a following stage, a portion of the power of the propagated light is derived as monitoring light, and this monitoring light then detected by means of a photodiode.
Several methods have been proposed for diverting the optical path of light that is propagated in a waveguide outside the waveguide plane. For example, a method is disclosed in Japanese Patent Laid-Open No. H4-155983 in which a (111) surface that has been formed by anisotropic etching of silicon is used to reflect upward light that is delivered from a waveguide. Alternatively, Japanese Patent Laid-Open No. H6-265738, Japanese Patent Laid-Open No. H11-326662, and Japanese Patent Laid-Open No. 2000-221347 disclose methods in which the exit end surface of a waveguide or any portion of a waveguide is etched at an angle and light is reflected in an upward direction with respect to the substrate on which the waveguide is formed. Japanese Patent Laid-Open No. H10-300961 discloses a method for fabricating a surface that reflects light in an upward direction by using a blade to directly cut the waveguide at 45 degrees and grinding the cut surface. Japanese Patent Laid-Open No. H7-159658 describes a method of mounting a prism inside the waveguide substrate and reflecting light in an upward direction. Still further, Japanese Patent No. 2687859 describes a technique in which a gold coat is applied to the surface of a spherical microlens, this lens is secured in the waveguide exit end surface through the use of a silicon etch-pit to reflect light in an upward direction. Finally, Japanese Patent Laid-Open No. 2000-189043 proposes a technique in which a micropyramid mirror, formed by transfer of silicon etch-pits, is mounted on a waveguide substrate to reflect light in an upward direction.
With the progress of multiplexing in wavelength-division multiplexing communication in recent years, the number of channels that are transmitted by waveguides in a single optical circuit device has also increased. For example, in an arrayed waveguide grating (AWG) that demultiplexes and multiplexes the wavelengths of light that is propagated in an optical fiber, the number of the input/output ports successively increases, in terms of the number of channels, from 8 channels and 16 channels to 40 channels, 80 channels, and 160 channels. When monitoring a light signal in each of the channels, a monitor method that employs a fiber coupler could be applied for up to 16 channels. With 40 or more channels, however, the routing of the optical fibers for monitoring after branching becomes extremely problematic, leading to an increase in the overall dimensions of the optical circuit device and an inability to adapt to the system needs. In addition, the high cost of fiber couplers entails the cost of a waveguide device having a monitor to increase in proportion to the number of channels. This poses serious problems, and developing a method of monitoring light that employs a method other than fiber couplers has therefore become a key issue.
Arrayed waveguides that apply to multichannel communication are generally formed on the same plane. Although it is possible to provide waveguide directional couplers between arrayed light-propagating waveguides and then derive the waveguide light and monitor light from the waveguide exit surfaces, photodiodes must be coupled to the ends of optical fibers after coupling the optical fibers. The routing of the optical fibers is therefore problematic.
As one means that can be considered for solving this problem, the monitor light can be diverted by waveguide directional couplers or Y-branching waveguides located between the arrayed waveguides, following which optical path conversion elements can be used to derive the monitor light from the plane on which the waveguides are formed.
In the method reported in the past of using a silicon etched surface for these optical path conversion elements, it is not possible to form silicon etch pits exclusively in portions between array waveguides to reflect light. A method can be considered in which a silicon anisotropic etched surface is cut for use as an optical path conversion element, but this approach is not practical due to problems regarding the number of processing steps, and the methods of mounting, securing, and positioning. Further, a method in which the waveguides are etched at an angle to cause light to reflect in a direction perpendicular to the substrate and a method of using a blade to directly cut the waveguide to 45 degrees and grinding the cut surface are also rendered impractical due to the extreme difficulty of processing exclusively the portions between waveguides that are formed in arrays. While a method may be employed in which microprisms are mounted between the waveguides, this method is impractical due to the extreme difficulty of forming microprisms of a size around 125 μm that is a distance between the waveguides, as well as to the demerit of cost. An optical path conversion element, proposed in Japanese Patent No. 2687859, in which a gold coat is applied to the surface of a microlens is difficult to apply, because a technique for applying a uniform gold coat to spherical grains on the order of 100 μm has not been established.
Further, when monitor light that is supplied from auxiliary waveguides provided between main waveguides is derived from waveguide exit surfaces together with propagated light, the fibers for the propagated light and the fibers for the monitor light must be alternately arranged in an array. In this case, since the diameter of the optical fibers that connect to the optical circuit device is 125 μm, the pitch of the array of the fibers for the propagated light cannot be reduced to 250 μm or less. In addition, the necessity for space to separate the fibers for propagated light and monitor light prevents the miniaturization of the monitor function part in an optical circuit device for multichannel communication.